Carbon fibers generally have excellent mechanical properties and especially high specific strength and specific modulus, and are therefore widely used as strength modifiers for various reinforcing materials in aerospace applications, leisure goods, industrial materials and the like. Because of their superior mechanical properties, they have potential application for reducing automobile weight and are receiving increasing attention as an important advance in solving the urgent problem of reducing carbon dioxide.
Such carbon fibers are produced by subjecting precursor organic polymer fibers to flame retardant treatment, firing and carbonization in the presence of oxygen. Various precursors may be mentioned, including cellulose, phenol resins, polyvinyl alcohol, vinylidene chloride, pitch, polyacrylonitrile (hereinafter abbreviated as “PAN”) and the like. Carbon fibers derived from PAN-based fibers are particularly superior in their dynamic properties such as specific strength and specific modulus, and because they can be produced with uniform and stable quality and performance, they are mass produced on an industrial scale.
When PAN-based fibers are subjected to flame retardant treatment followed by carbonization to produce carbon fibers, it has normally been necessary to carry out heat treatment for a long period in a high-temperature oxidizing atmosphere of 200-400° C., as the conditions for flame retardant treatment. This is because flame retardant treatment of the precursor PAN-based fibers attempted all at once in a short period at a temperature of 500° C. or above produces a sudden exothermic decomposition reaction which results in self combustion and decomposition of the polymer, preventing formation of the desired carbon skeleton. Furthermore, prolonged high-temperature heat treatment is not only problematic in economic terms because of high energy consumption and low productivity, but also in terms of quality from the standpoint of strength reduction due to fusion between single fibers, and in terms of process flow since filament breakage readily occurs at high temperature, for which reasons a need exists for industrial improvement.
Various proposals have been set forth in the prior art in order to avoid such problems. For example, there has been proposed the use of a PAN-based precursor obtained by copolymerization of a specific amount of a polymerizable unsaturated carboxylic acid ammonium salt (for example, see patent documents 1 and 2) and the use of a precursor which is PAN obtained by copolymerization of a long-chain alkyl ester of a polymerizable unsaturated carboxylic acid (for example, see patent document 3).
These precursors exhibit certain effects of promoting flame retardant reaction, but the low copolymerizability of unsaturated carboxylic acids often leads to blocking of the copolymer. In addition, a high proportion of a carboxylic acid component with poor heat resistance is a disadvantage in that it can lead to lower yields as a result of thermal decomposition during the flame retardant step following the polymerization step.
On the other hand, it has been demonstrated that copolymerization of α-chloroacrylonitrile with acrylonitrile can drastically shorten the flame retardant time and solve the problem of poor productivity (for example, see patent documents 4 and 5). Still, a large amount of the costly α-chloroacrylonitrile component must be used for copolymerization in order to adequately shorten the flame retardant time, thus presenting an economic drawback which counteracts with the improvement in productivity.
It has also been disclosed that using a terpolymer incorporating itaconic acid and an acrylamide-based monomer with acrylonitrile can improve the flame retardant properties (for example, see patent document 6), but in addition to the difficulty of obtaining a homogeneous copolymer with three different monomers, any excess of itaconic acid can result in a violent exothermic reaction producing damage in the fiber structure, while an excess of the acrylamide monomer can produce fiber fusion and can thereby complicate control of the copolymer composition and influence productivity. Other proposals include using hydroxymethylene (for example, see patent document 7), halogenated alkyl esters of unsaturated carboxylic acids (for example, see patent document 8) and silicon- or fluorine-containing unsaturated monomers (for example, see patent document 9) as copolymerization components, but none of these have exhibited satisfactory effects from a cost and performance standpoint.
On the other hand, research is also progressing in the area of flame retardant reaction for PAN-based fibers. For example, flame retardant reaction for PAN-based fibers is now known to be initiated by oxidation and cyclization of adjacent nitrile skeletons (for example, see non-patent document 1).
In addition, it has been reported through past research that for such thermally induced reactions, the polymer microstructure, and specifically the stereoregularity of the polymer main chain described by its tacticity, can affect the reaction temperature and reaction rate. For example, it has been demonstrated that formation of an imine skeleton from nitrile groups by heating proceeds preferentially at low temperature with isotactic chains rather than with atactic or syndiotactic chains (for example, see non-patent documents 2 and 3).
Copolymers of PAN with no stereostructure regularity, i.e. atactic PAN, obtained by ordinary radical polymerization have been used as conventional carbon fiber precursor polymers and carbon fiber precursors. Still, no literature or reports have been published to date which examine the use of a single PAN with stereostructure regularity, i.e. isotactic PAN, as a carbon fiber precursor polymer and carbon fiber precursor with excellent flame retardant reactivity.
[Patent Document 1]
Japanese Unexamined Patent Publication SHO No. 48-63029
[Patent Document 2]
Japanese Examined Patent Publication SHO No. 58-48643
[Patent Document 3]
Japanese Unexamined Patent Publication SHO No. 61-152812
[Patent Document 4]
Japanese Examined Patent Publication SHO No. 49-14404
[Patent Document 5]
Japanese Examined Patent Publication HEI No. 6-27368
[Patent Document 6]
Japanese Unexamined Patent Publication HEI No. 11-117123
[Patent Document 7]
Japanese Unexamined Patent Publication SHO No. 52-53995
[Patent Document 8]
Japanese Unexamined Patent Publication SHO No. 52-55725
[Patent Document 9]
Japanese Unexamined Patent Publication HEI No. 2-14013
[Non-patent Document 1]
W. Watt et al., “Proceedings of the International Carbon Fiber Conference London”, Paper No. 4, 1971
[Non-patent Document 2]
N. A. Kobasova et al., “VYSOKOMOLEKULYAR NYE SOEDINENIYA SERIYA A”, Russia, 13(1), 1971, P. 162-167
[Non-patent Document 3]
M. A. Geiderikh, “VYSOKOMOLEKULYAR NYE SOEDINENIYA SERIYA A”, Russia, 15(6), 1973, P. 1239-1247.